Rotation sensor with onboard power generation

ABSTRACT

A rotation sensor configured to be mounted on a rim of a wheel. The rotation sensor includes a band sized and shaped to fit around the rim of the wheel, a first element mounted on the band that generates a first time-varying electrical signal in response to a rotational movement, a second element mounted on the band that generates a second time-varying electrical signal in response to the rotational movement, a processor mounted on the band that receives the first and second time-varying electrical signals and processes the first and second time-varying electrical signals to determine a rotational speed, and a rechargeable power source that receives the first and second time-varying electrical signals, consumes at least a portion of the first and second time-varying electrical signals to recharge the rechargeable power source, and generates a power signal. The processor is connected to the rechargeable power source to receive the power signal.

BACKGROUND

The present invention relates to rotation sensors. More particularly,the invention relates to rotation sensors that detect wheel rotation,from which the speed of a vehicle can be determined.

Some rotation sensors include components (often targets) in the wheel orwheel rim and other components (that process information from thetarget) that are located on the chassis. The rotation sensors determinethe time it takes for targets to pass the sensor. In some technologies,the angular separation of the targets and the elapsed time is used todetermine the speed of the wheel.

SUMMARY

A number of challenges are created by the location of the targets, whichare in or on a rotating member (e.g., the wheel), and the sensingelements, which are in or on a non-rotating member (e.g., the chassis).First, since there is relative motion between the components, a simplewired connection between the two can not be used. Either a slip ring (orsimilar connector) or a wireless connection must be used. Second, inmany instances, the placement of the sensor components exposes them tothe environment (e.g., water, snow, cold, dirt, dust, stones, rocks, andthe like.). The sensor components may also be exposed to heat from thevehicle brakes. A third challenge relates to providing power to thesensor components. Components located on a vehicle chassis can, in manyinstances, be connected to a vehicle power system. However, providingpower to sensor elements located on rotating components is difficult,because, as was noted, a simple wire connection can not be used betweena rotating component (e.g., a sensor target) and a stationary component(e.g., the vehicle power system). Currently, many sensor components arebattery-powered (by a battery that is separate from the vehicle battery)to avoid having to transmit power from the vehicle power system to therotating sensor element. To meet the goals of vehicle manufacturers,such elements must operate for 100,000 miles or 10 years. However, manybatteries are not capable of meeting this requirement.

To overcome at least some of these disadvantages, the inventors havedeveloped a technology where the rotation sensor is located entirely inthe wheel. The rotation sensor includes a rechargeable power source(e.g., conversion equipment and a battery) and an onboard powergenerator that recharges the storage device. The storage device providespower to a microprocessor and wireless transmitter. The rotation sensorincludes no sensing elements that require power. Rather, the rotationsensor includes power-generating elements that generate a voltage orsignal when subjected to mechanical deformation, such as bending. Themicroprocessor receives the signals produced by the power-generatingelements. The power generating elements have a dual function and alsoact as sensing elements. The microprocessor processes the signals fromthe power-generating elements to determine rotation information.Ultimately, this information is used to determine vehicle speed. Thesignals from the power-generating devices are also provided to therechargeable power source.

In one embodiment, the invention provides a rotation sensor configuredto be mounted on a rim of a wheel. The rotation sensor includes a band,sized and shaped to fit around the rim of the wheel. A first element, asecond element, and a processor are mounted on the band. The firstelement generates a first time-varying electrical signal in response toa rotational movement. The second element generates a secondtime-varying electrical signal in response to the rotational movement.The processor receives the first and second time-varying electricalsignals and processes them to determine a rotational speed. The rotationsensor also includes a rechargeable power source that receives the firstand second time-varying electrical signals. The rechargeable powersource consumes at least a portion of the first and second time-varyingelectrical signals to recharge the rechargeable power source. The powersource also outputs a power signal to the processor.

In another embodiment, the invention provides a rotation sensing systemfor determining a rotational speed of a wheel of a vehicle. The rotationsensing system includes a wheel that rotates with respect to thevehicle, and the wheel includes a rim. The rim is substantiallycylindrically shaped with an inner surface and an outer surface, has asubstantially circular cross-sectional area, and is operable to rotateabout an axis that passes substantially through a center of thesubstantially circular cross-sectional area. A tire surrounds the rim,and the tire and the rim form an airtight space therebetween. Therotation sensing system also includes a rotation sensor coupled to theouter surface of the rim and positioned in the airtight space. Therotation sensor includes two sensing elements (a first element and asecond element). Each element is positioned on the outer surface of therim and generates a time-varying electrical signal in response torotation of the wheel. A processor receives the time-varying signalsfrom the elements and processes the time-varying signals to determinethe rotational speed. The rotation sensing system also includes arechargeable power source that provides power to the processor. Therechargeable power source takes the form of or includes a power storagedevice and receives the first and second time-varying electrical signalsto recharge the power storage device.

In another embodiment, the invention provides a method of sensing anangular speed of a wheel of a vehicle. The method includes generating afirst time-varying signal with a first element in response to a rotationof the wheel, generating a second time-varying signal with a secondelement in response to the rotation of the wheel, providing at least oneof the first time-varying signal and the second time-varying signal to arechargeable power source to charge the rechargeable power source,providing the first time-varying signal and the second time-varyingsignal to a processor, providing a power signal to the processor, andcomparing the first time-varying signal and the second time-varyingsignal to determine a difference between the first time-varying signaland the second time-varying signal, the difference at least partiallyindicative of a rotational speed of the wheel.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a wheel illustrating one mechanism formounting a rotation sensor on a rim.

FIG. 2 is a sectional view of the wheel of FIG. 1 and illustrates arotation sensor.

FIG. 3 is a schematic illustrating one embodiment of a circuit for therotation sensor of FIG. 2.

FIG. 4 is a schematic illustrating a second embodiment of a circuit thatmay be used with the rotation sensor of FIG. 2.

FIG. 5 is a schematic illustrating four possible positions of sensingelements on the rim.

FIG. 6 is a flowchart of one embodiment of logic that a processor mayuse to process signals received from the sensing elements.

FIG. 7 is a continuation of the flowchart of FIG. 6.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

FIG. 1 is an exploded view of a wheel 10 having a rim 14. The rim 14 isdesigned to have a tire 18 mounted thereon. When the tire 18 is mountedon the rim 14 a defined, toroidally-shaped space is created between thetire 18 and the rim 14. A rotation sensor 19 is mounted on the rim 14within the space between the rim 14 and the tire 18. As best seen byreference to FIG. 3, the rotation sensor 19 includes a plurality ofsensing elements 26, 30, 34, and 38, a microprocessor 42, a rechargeablepower source 46, and a wireless transmitter 50.

Referring back to FIG. 1, the components of the rotation sensor 19 aremounted on a band 22 that is fitted to the rim 14 and secured thereto tosubstantially prevent movement of the rotation sensor 19 with respect tothe rim 14. The band 22 may be formed of an elastic material, metal,plastic, etc. The band 22 may be secured in place by for example, anadhesive, tightening of the band 22 around the rim 14, a combination ofboth, or other suitable means. In other embodiments, the rotation sensor19 may not include a band 22 and the components may be mounted directlyto the rim 14 via, for example, soldering, adhesive, pins, or othersuitable means.

Preferably, the rotation sensor 19 is configured for easy attachment andremoval from the rim 14 such as by a band 22 that may be tightenedaround and loosened from the rim. A suitable band 22 may fasten with,for example, hook-and-loop fasteners or other fasteners that can betightened or loosed with hand tools. Thus, when the rim 14 and tire 18are replaced during the lifetime of the vehicle, the rotation sensor 19may be removed from the rim 14 by loosening the band 22. The band 22 androtation sensor 19 may then be placed around a new rim and tightened tosecure the rotation sensor in position before a new tire is mounted onthe rim. Due to the variety of aftermarket rims and tires, which areavailable in different designs and sizes, a removably mounted rotationsensor 19 is desirable so it can be removed from an original rim 14 andmounted on a desired aftermarket rim.

As noted, the rotation sensor 19 includes a plurality of sensingelements and, in the embodiment illustrated in FIG. 3, the sensorincludes four elements: 26, 30, 34, and 38. The sensing elements 26, 30,34, and 38 are piezoelectric elements that produce a voltage whendeformed. The magnitude of the voltage varies with the amount ofdeformation of the piezoelectric element 26, 30, 34, and 38. Thepolarity of the voltage produced varies with the direction of thedeformation. The voltage produced may vary in time if the piezoelectricelement 26, 30, 34, and 38 is subjected to deformations that vary intime. Thus, the voltages produced in time may be referred to as aninformation signal because the time-varying changes may provideinformation about the rim 14 (such as forces that are acting upon it, asis discussed in greater detail below). In other embodiments, therotation sensor 19 may include as little as two sensing elements or mayinclude more than four sensing elements. It is preferable that therotation sensor 19 include an even number of sensing elements positionedopposite each other along the circumference of the rim 14 such thatpairs of elements may be identified. For example, as illustrated in FIG.5, the first sensing element 26 and the third sensing element 34 formone sensing pair and the second sensing element 30 and the fourthsensing element 38 form a second sensing pair.

The microprocessor 42 receives and processes information signals fromthe sensing elements 26, 30, 34, and 38. The microprocessor 42 mayprocess the information signals according to a predefined logic, asillustrated in FIGS. 6-8, to determine information about the wheel 10that may be used to determine wheel speed. In the present embodiment,the microprocessor 42 determines a wheel speed for every quarter turn(90 degrees of rotation) of the wheel 10.

As best seen by reference to FIG. 4, the rechargeable power source 46(which may take the form of a storage device such as a rechargeablebattery) is connected to the microprocessor 42 and provides power to themicroprocessor 42. Each sensing element 26, 30, 34, and 38 is connectedin parallel across the battery 46 and the microprocessor 42. The battery46 is configured to receive the signals produced by the sensing elements26, 30, 34, and 38 and use those signals to recharge the battery 46. Insome embodiments, the signals generated by the sensing elements 26, 30,34, and 38 are processed in a conditioning circuit (e.g., rectifyingdiodes 66, 70, 72, and 76), as illustrated in FIG. 4, and then providedto the battery 46 as a power signal. Thus, no additional power source isrequired for the rotation sensor 19 to operate. The battery 46 may berecharged during normal use of the sensor 10.

As illustrated in FIG. 3, the wireless transmitter 50 communicates witha vehicle electronic control unit (ECU) 62 of the vehicle. Themicroprocessor 42 processes the information signals received from thesensing elements 26, 30, 34, and 38 and determines a wheel speed. Thetransmitter 50 wirelessly communicates with a receiver 58 to send thewheel speed to the vehicle ECU 62. The ECU 62 may use the wheel speedinformation in other systems, such as speedometers, vehicle stabilitycontrol systems, and traction control systems. The transmitter 50 mayalso transmit the output signal to other devices that require rotationinformation.

FIG. 5 schematically illustrates placement of the sensing elements 26,30, 34, and 38 on the rim 14. For convenience, the positions of thesensing elements 26, 30, 34, and 38 will be referred to as 0 degrees, 90degrees, 180 degrees, and 270 degrees. For example, in position A thefirst sensing element 26 is at 0 degrees, the second sensing element 30is at 90 degrees, the third sensing element 34 is at 180 degrees, andthe fourth sensing element 38 is at 270 degrees. In position B, thewheel (i.e., tire 18 and rim 14) has rotated 90 degrees clockwise withrespect to position A. In position C, the wheel has rotated 180 degreesclockwise with respect to position A. In position D, the wheel hasrotated 270 degrees clockwise with respect to position A. Thus, eachsensing element 26, 30, 34, and 38 rotates 90 degrees clockwise fromposition A to position B, from position B to position C, from position Cto position D, and from position D to position A.

Signals produced by each sensing element 26, 30, 34, and 38 vary withthe position of the sensing element. Gravity acts on the tire 18, rim14, and sensing elements 26, 30, 34, and 38 in the direction of thearrow G shown FIG. 5. When the wheel 10 is stationary and in position A,the first sensing element 26 is oriented horizontally, whereby gravitybends the first sensing element 26 toward the center of the rim 14,which causes the sensing element 26 to output a positive voltage. Thethird sensing element 34, also oriented horizontally, bends away fromthe center of the rim 14 due to the force of gravity and outputs anegative voltage. The second and fourth sensing elements 30 and 38output substantially zero voltage because the second and fourth sensingelements 30, 38 are oriented vertically and the force of gravity doesnot cause either the second or fourth sensing elements 30 and 38 tobend.

As the wheel 10 turns, the sensing elements 26, 30, 34, and 38 arepositioned as shown in position B of FIG. 5. In this position, thefourth sensing element 38 outputs a positive voltage, the second sensingelement 30 outputs a negative voltage, and the first and third sensingelements 26, 34 output substantially zero voltage. Similarly, when thewheel 10 turns another 90 degrees, the sensing elements 26, 30, 34, and38 are positioned as shown in position C, and after another 90 degrees,the sensing elements 26, 30, 34, and 38 are positioned as shown inposition D. In general (and when considering gravity alone), the sensingelement in the 0 degree position outputs a positive voltage, the sensingelement in the 180 degree position outputs a negative voltage, and thesensing elements in the 90 degree and 270 degree positions output asubstantially zero voltage. Thus, as the wheel 10 rotates, the outputsof the sensing elements 26, 30, 34, and 38 change. As discussed ingreater detail below, forces other than gravity may act on the sensingelements 26, 30, 34, and 38.

During rotation of the wheel 10, centrifugal forces are exerted on thesensing elements 26, 30, 34, and 38, changing the outputs of the sensingelements 26, 30, 34, and 38. During driving, other events may causeother forces to be exerted on the sensing elements 26, 30, 34, and 38.The forces may be generated as a result of traveling over bumps,braking, acceleration, collisions, etc. As will be discussed below,these events affect the information signals received by themicroprocessor 42 and are accounted for during the processing of thesignals.

FIGS. 6 and 7 illustrate one example of logic used by the microprocessor42 to determine wheel speed in rotations per minute (rpm) from thesignals received from the sensing elements 26, 30, 34, and 38. Ingeneral, the microprocessor 42 detects 90 degree increments of rim 14rotation and an associated elapsed time in seconds (sec). From thatinformation, the microprocessor 42 calculates wheel speed using thefollowing equation:

$\begin{matrix}{{{Wheel}\mspace{14mu}{Speed}\mspace{14mu}({rpm})} = \frac{60}{4\left( {{Elapsed}\mspace{14mu}{Time}\mspace{14mu}\left( \sec \right)} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

For simplicity, the output voltage due to gravity that is produced by asensing element oriented in the 0 degree position is assigned anarbitrary value of +1 g, and the output voltage due to gravity that isproduced by a sensing element oriented in the 180 degree position isassigned an arbitrary value of −1 g. Sensing elements positioned in the90 and 270 degree positions output voltages due to gravity that areassigned values of 0 g. Centrifugal forces due to the turning of thewheel are applied substantially evenly, in a radially outward direction,on the sensing elements as the wheel 10 rotates and cause the sensingelements to each output a more negative voltage.

FIGS. 6 and 7 illustrate the logic used by the microprocessor 42. Ofcourse, in other embodiments, the microprocessor 42 can use a differentlogic. The logic illustrated in FIGS. 6 and 7 is used for exemplarypurposes and is not intended to limit the present invention.

At block or step 100 of FIG. 6, the microprocessor 42 receives theinformation signals corresponding to the sensing elements 26, 30, 34,and 38 and records a current time t_(n) when the information isreceived. The current time t_(n) corresponds to an internal clock of themicroprocessor 42.

At block or step 104, the microprocessor 42 converts the informationsignals corresponding to the sensing elements 26, 30, 34, and 38 intovalues S1, S2, S3, and S4, respectively, wherein each value has a unitof g. Thus, S1 represents the information signal corresponding tosensing element 26 in units of g, S2 represents the information signalcorresponding to sensing element 30 in units of g, S3 represents theinformation signal corresponding to sensing element 34 in units of g,and S4 represents the information signal corresponding to sensingelement 38 in units of g.

There are six possible pairs that can be formed from the values S1, S2,S3, and S4. The absolute values of the differences between the values ofeach pair are calculated at block or step 108 and include |S1-S3|,|S2-S4|, |S1-S2|, |S1-S4|, |S3-S2|, and |S3-S4|. The absolute values canbe subdivided into a first and second group. The first group includesthe values that correspond to sensors positioned opposite each other, or180 degrees apart. Thus, the first group includes the absolute values|S1-S3| and |S2-S4|. The second group includes the other pairs ofvalues, namely, the absolute values |S1-S2|, |S1-S4|, |S3-S2|, and|S3-S4|.

It was empirically determined that at a point in time, the absolutevalues in the second group are all equal to each other when there is nohorizontal acceleration of the wheel 10. However, when the wheel 10experiences horizontal acceleration, the absolute values in the secondgroup are not all equal to each other. More specifically, the values|S1-S2| and |S3-S4| are equal to each other, and the values |S1-S4| and|S2-S3| are equal to each other but different from the values of |S1-S2|and |S3-S4|. Thus, the absolute values of the second group |S1-S2|,|S1-S4|, |S3-S2|, and |S3-S4| are compared to each other at block orstep 112 to determine if horizontal acceleration is present (block orstep 116) or absent (block or step 120). When the wheel 10 does notexperience horizontal acceleration, the microprocessor 42 defines athreshold TH as 2|S1-S2|g.

When the wheel 10 experiences horizontal acceleration, withoutexperiencing any vertical acceleration, then there is no effect on thevalues corresponding to the sensors positioned in the 0 and 180 degreepositions. Thus, the absolute difference between the valuescorresponding to the sensors positioned in the 0 and 180 degreepositions is equal to 2 g. For example, when the rotation sensor 19 isorientated as shown in position A of FIG. 3 and assuming each sensingelement 26, 30, 34, and 38 experiences a centrifugal force in units of g(Cg), the microprocessor 42 converts the information signals into (1−C)gfor the first sensing element 26, and (−1−C)g for the third sensingelement 34 at block or step 104. Then, the absolute value of thedifference between the first sensing element 26 and the third sensingelement 34 is equal to |S1-S3|=|(1−C)g−(−1−C)g|=2 g. Regardless of theamount of centrifugal force Cg experienced by each of the sensingelements 26, 30, 34, and 38, the absolute difference between the sensingelements positioned in the 0 and 180 degree positions is substantiallyequal to 2 g.

However, when the wheel 10 experiences both horizontal and verticalacceleration, all of the values S1, S2, S3, and S4 are affected and thevalues corresponding to the sensors positioned in the 0 and 180 degreepositions are not equal to 2 g. Thus, at block or step 128, themicroprocessor 42 compares the values in the first group to determinethe presence or absence of vertical motion. Specifically, themicroprocessor 42 determines if |S1-S3|=2 g or if |S2-S4|=2 g (block orstep 128). When no vertical acceleration is detected (block or step132), the microprocessor 42 defines the threshold TH to be equal to 2 g.When the microprocessor 42 determines that vertical acceleration ispresent (block or step 140), the microprocessor 42 calculates andcompares a predefined set of values to determine an appropriatethreshold value.

The predefined set of values was determined empirically and includes thefollowing eight values: 2|S1-S2|±|S2-S4|, 2|S1-S4|±|S2-S4|,2|S1-S2|±|S1-S3|, 2|S1-S4|±|S1-S3|. The microprocessor 42 identifiesequal value pairs from the results, at block or step 144. Of the equalvalue pairs identified, the microprocessor 42 determines the greatestabsolute value (block or step 152) and assigns it to the threshold TH(block or step 154).

At block or step 158, the microprocessor 42 determines if |S1-S3|=TH. Ifyes, the microprocessor 42 assumes that the sensing elementcorresponding to S1 and the sensing element corresponding to S3 arepositioned in the 0 and 180 degree positions. If no, the microprocessor42 determines if |S2-S4|=TH (block or step 166). If no, then neither thesensing elements corresponding to the values S1 and S3 nor the sensingelements corresponding to the values S2 and S4 are in the 0 and 180degree positions. Thus, the microprocessor 42 determines an error anddisregards the information signals received. If |S2-S4|=TH, then themicroprocessor 42 knows that the sensing elements corresponding to thevalues S2 and S4 are in the 0 and 180 degree positions.

After the microprocessor 42 determines whether the sensing elementscorresponding to the values S1 and S3 are positioned in the 0 and 180degree positions (block or step 162) or the sensing elementscorresponding to the values S2 and S4 are positioned in the 0 and 180degree positions (block or step 174), the microprocessor 42 determineswhether the wheel rotated 90 degrees, as shown at block or step 178. Themicroprocessor 42 compares the current state to the previous state todetermine whether the wheel 10 rotated 90 degrees. If the wheel 10 didnot rotate 90 degrees, the data is discarded. If the wheel 10 did rotate90 degrees, the microprocessor 42 calculates the elapsed timet_(n)-t_(n−1), where t_(n−1) is the time at which one sensing elementpair is in the 0 and 180 degree positions and t_(n) is the time at whichthe other sensing element pair is in the 0 and 180 degree positions(block or step 186). The microprocessor 42 uses the elapsed time andEquation 1 to calculate the wheel speed, as shown at block or step 190,for the current time period. The wheel speed is wirelessly transmittedto the vehicle ECU 62 or other vehicle systems, as described above, forfurther processing.

Thus, the invention provides, among other things, a rotation sensor thatdetermines rotational information about a wheel 10 mounted on a vehicle.Various features and advantages of the invention are set forth in thefollowing claims.

1. A rotation sensor configured to be mounted on a rim of a wheel, therotation sensor comprising: a band sized and shaped to fit around therim of the wheel; a first element mounted on the band that generates afirst time-varying electrical signal in response to a rotationalmovement; a second element mounted on the band that generates a secondtime-varying electrical signal in response to the rotational movement; aprocessor mounted on the band that receives the first and secondtime-varying electrical signals from the first and second elementsrespectively, and processes the first and second time-varying electricalsignals to determine a rotational speed; and a rechargeable power sourcethat receives the first and second time-varying electrical signals fromthe first and second elements respectively, consumes at least a portionof the first and second time-varying electrical signals to recharge therechargeable power source, and generates a power signal, and wherein theprocessor is connected to the rechargeable power source to receive thepower signal.
 2. The rotation sensor of claim 1, further including atransmitter that transmits the rotational speed to a second processor.3. The rotation sensor of claim 1, wherein the first element and thesecond element are mounted on the band substantially 180 degrees apartand the processor processes the first and second time-varying electricalsignals to determine an amount of time for the wheel to rotate 180degrees.
 4. The rotation sensor of claim 1, wherein the first and secondelements are piezoelectric elements.
 5. The rotation sensor of claim 1,further comprising a third element mounted on the band that produces athird time-varying electrical signal in response to the rotationalmovement and a fourth element mounted on the band that produces a fourthelectrical time-varying signal in response to the rotational movement,and wherein the processor receives the third and fourth time-varyingelectrical signals and processes the third and fourth time-varyingsignals to determine the rotational speed.
 6. The rotation sensor ofclaim 5, wherein the first element and the second element are mounted onthe band substantially 180 degrees apart, the third element is mountedon the band substantially 90 degrees apart from the first element and 90degrees from the second element, and the fourth element is mounted onthe band substantially 90 degrees apart from the first element, 90degrees apart from the second element, and 180 degrees from the thirdelement, and wherein the processor processes the first and secondtime-varying electrical signals and the second and third time-varyingelectrical signals to determine the rotational speed.
 7. The rotationsensor of claim 6, wherein the processor processes the first and secondtime-varying electrical signals and the third and fourth time-varyingelectrical signals to determine an occurrence of an acceleration of thewheel.
 8. The rotation sensor of claim 6, wherein the processorprocesses the first and second time-varying electrical signals and thethird and fourth time-varying electrical signals to determine anoccurrence of a force acting on the wheel due to an acceleration of thewheel.
 9. The rotation sensor of claim 5, wherein the rechargeable powersource receives the first and second time-varying electrical signals andthe third and fourth time-varying electrical signals, consumes at leasta portion of the first and second time-varying electrical signals andthe third and fourth time-varying electrical signals to recharge therechargeable power source.
 10. A rotation sensing system for determininga rotational speed of a wheel of a vehicle, the rotation sensing systemcomprising: a wheel that rotates with respect to the vehicle, the wheelincluding: a rim that is substantially cylindrically shaped with aninner surface and an outer surface, the rim having a substantiallycircular cross-sectional area, and the rim operable to rotate about anaxis that passes substantially through a center of the substantiallycircular cross-sectional area; and a tire that surrounds the rim, thetire and the rim forming an airtight space therebetween; a rotationsensor coupled to the outer surface of the rim and positioned in theairtight space, the rotation sensor including: a first elementpositioned on the outer surface of the rim that generates a firsttime-varying electrical signal as the wheel rotates; a second elementpositioned on the outer surface of the rim substantially opposite thefirst element that generates a second time-varying electrical signal asthe wheel rotates; a processor that receives the first time-varyingelectrical signal from the first element, and the second time-varyingelectrical signal from the second element, the processor processes thefirst time-varying signal and the second time-varying signal todetermine the rotational speed; and a rechargeable power source thatprovides power to the processor and receives the first time-varyingelectrical signal from the first element, and the second time-varyingelectrical signal from the second element, to recharge the rechargeablepower source.
 11. The rotation sensing system of claim 10, furtherincluding a transmitter that transmits the rotational speed to a vehiclecontrol unit.
 12. The rotation sensing system of claim 10, wherein thefirst element, the second element, the processor, and the rechargeablepower source are mounted on a band configured to fit around the rim ofthe wheel, the band removably mounted on the outer surface of the rim.13. The rotation sensor of claim 10, wherein the first element and thesecond element are mounted on the rim substantially 180 degrees apartand the processor processes the first and second time-varying electricalsignals to determine an amount of time for the wheel to rotate 180degrees.
 14. The rotation sensing system of claim 10, wherein the firstelement and the second element are piezoelectric elements.
 15. Therotation sensing system of claim 10, further comprising a third elementpositioned on the outer surface of the rim between the first element andthe second element that generates a third time-varying electrical signalas the wheel rotates, and a fourth element positioned on the outersurface of the rim substantially opposite the third element thatgenerates a fourth time-varying electrical signal as the wheel rotates,wherein the processor receives the third time-varying electrical signaland the fourth time-varying electrical signal and processes the thirdand fourth time-varying electrical signals to determine the rotationalspeed.
 16. The rotation sensing system of claim 15, wherein therechargeable power source receives at least one of the thirdtime-varying electrical signal and the fourth time-varying electricalsignal to recharge the rechargeable power source.
 17. The rotationsensing system of claim 15, wherein the processor processes the firstand second time-varying electrical signals and the third and fourthtime-varying electrical signals to determine an occurrence of anacceleration of the wheel in at least one of a horizontal direction anda vertical direction.
 18. The rotation sensor of claim 15, wherein theprocessor processes the first and second time-varying electrical signalsand the third and fourth time-varying electrical signals to determine anoccurrence of a force acting on the wheel due to an acceleration of thewheel.
 19. A method of sensing an angular speed of a wheel of a vehicle,the method comprising: generating a first time-varying signal with afirst element in response to a rotation of the wheel; generating asecond time-varying signal with a second element in response to therotation of the wheel; providing at least one of the first time-varyingsignal from the first element, and the second time-varying signal fromthe second element, to a rechargeable power source to charge therechargeable power source; providing the first time-varying signal andthe second time-varying signal to a processor; providing a power signalto the processor from the rechargeable power source; and comparing thefirst time-varying signal and the second time-varying signal todetermine a first difference between the first time-varying signal andthe second time-varying signal, the first difference at least partiallyindicative of a rotational speed of the wheel.
 20. The method of claim19, further including generating a third time-varying signal with athird element in response to the rotation of the wheel; generating afourth time-varying signal with a fourth element in response to therotation of the wheel; providing at least one of the third time-varyingsignal and the fourth time-varying signal to the rechargeable powersource to charge the rechargeable power source; providing the thirdtime-varying signal and the fourth time-varying signal to the processor;comparing the third time-varying signal and the fourth time-varyingsignal to determine a second difference between the third time-varyingsignal and the fourth time-varying signal, the second difference atleast partially indicative of the rotational speed of the wheel; anddetermining the rotational speed of the wheel by processing the firstdifference and the second difference.